The thermoelastic properties of shape memory alloys (SMA) have been known since the 1930s. Experimental use of SMAs continued for decades. Commercially viable uses for SMAs remained elusive until the 1990s. Today, SMA materials are finding unique applications in a variety of industries from the automobile industry to the medical industry. One application is the use of SMA materials for actuators.
While SMA actuators have been attempted, existing SMA actuators suffer from a number of shortcomings. For example, many existing SMA actuators have insufficient protective mechanisms to ensure safe, reliable and extended lifetime operation of the SMA materials employed in the actuator. Because of the special properties of the SMA materials currently used, there is a need to prevent excessive force, or excessive electrical power from being applied to the SMA element for extended periods. For example, SMA materials are commonly used in wire form typically with small diameter wires. The advantageous austenitic contraction of an SMA material is quite large even in a small diameter wire. However, while SMA wires typically have high material strength they have low ultimate tensile strength. As such, particular care should be taken in designs using SMA wires. SMA wire failure may result in catastrophic failure of the overall actuator. There is a need, therefore, for improved SMA-based actuators that overcome the shortcomings of the existing SMA actuators.